There is a great commercial need for stringent quality control of crystalline platings, films, coatings, and coating processes. Stresses and/or discontinuities in metal or ceramic coatings may lead to cracking, corrosion, peeling, or a myriad of other problems. More importantly, significant manufacturing cost reductions and improvements in quality and reliability can be achieved by insuring that 1) the applied coating thickness is not excessive for a specific application, and 2) the elemental and phase composition as well as the crystallite size and orientation are correct. Tighter control over the coating thickness and composition conserves raw material, and the cost savings can be great when the coating contains gold, silver, or other rare material.
Unfortunately, the above-described quality control of platings, films, and coatings which are electroplated, hot dipped, chemical vapor deposited, etc., is very difficult. (Note: for purposes of this application the terms "plating," "film," and "coating" will be used interchangeably).
The existing technique of X-ray fluorescence (XRF) e.g. U.S. Pat. No. 5,137,727 by F. Vogler does provide a limited capability for analyzing elemental composition and thickness, but this method is sometimes slow and unsuited for many coatings. Specifically, when other underlying platings or the substrate itself contain one of the electroplated alloy elements, XRF analysis yields ambiguous results. This is often the case with coatings such as Pd:Ni alloy. Hence, XRF analysis may be inadequate. Moreover, XRF does not provide information on phase composition, strains, or crystallite size, and orientation.
Whenever X-rays encounter a crystalline material, the regularly spaced atoms of the crystal diffract some of the X-rays. The characteristic diffraction pattern is indicative of the crystal structure of the material, and various properties of the material can be analyzed based upon particular features of the pattern. X-ray diffraction (XRD) analysis evolved from this basic premise, and XRD has proven to be a practical, non-destructive way of gaining a more complete analysis than is possible with, for example, XRF.
The basis for all XRD techniques is the Bragg relation, which equates the interplanar atomic spacing (d) of a material to the Bragg angle (.theta.). The relation is as follows: EQU n.lambda.=2dsin.theta.
where,
n is usually unity for polycrystalline XRD,
.lambda. is the wavelength of diffracting X-rays,
d is the interplanar spacing of a particular Miller index, and
.theta. is the Bragg angle.
By comparing a measured diffraction pattern to standard patterns cataloged in a database, the material can be identified. Hence, XRD analysis holds great potential for on-line monitoring and quality control, and XRD has been used extensively to determine second phase composition in steels, alloy content in platings, and preferred orientation (texture) in copper alloy strip and aluminum sheet. XRD has also been used to measure residual stresses and texture in various metals including nickel platings. Also, XRD has been applied to texture assessment in steel sheet (in Germany) and to aluminum sheet stock (in the U.S.A.), in both instances using an energy dispersive X-ray detector. With a diffractometer, X-rays of predetermined wavelength are emitted from a source and are diffracted from a sample as shown in FIG. 1. The intensity of the diffracted X-rays is sensed by a detector which is moved continuously or step-wise along the diffraction angle. The sample or source is rotated (or "scanned") over one-half the diffraction angle (i.e., twice the Bragg angle .theta.) to determine a diffraction pattern such as the exemplary NaCl pattern shown in FIG. 2. More recently, position sensitive detectors have been substituted for conventional detectors in some applications to eliminate the scanning. In either case, the resulting diffraction pattern may be used to identify the phase composition of the sample..
Originally, diffraction methods including XRD were possible only in the laboratory. As with XRF, the necessary XRD equipment was large, unwieldy, and required a great deal of time to operate. A complete scan took upward of ten minutes. This was unsuitable for on-line monitoring and quality control because it could not be used for real-time measurements.
A solution to the above-described problem became possible through the development of "position sensitive detectors" (PSDs). These detectors allow simultaneous measurement of diffracted X-rays over a range of .theta.. Using a PSD, a two-dimensional intensity-position snap-shot can be obtained quickly, and no mechanical scanning operation is necessary.
Further progress toward on-line compositional analysis was made with the development of a Position Sensitive Scintillation Detector (PSSD). This device combines fiber optic and electrooptical technology into a unique position sensitive detector. See, Ruud, "X-Ray Analysis and Advances in Portable Field Instrumentation," Journal of Metals, pp. 10-15 (June 1979), Ruud, "A Unique Position Sensitive Detector For X-Ray Powder Diffraction," Industrial Research and Development, pp. 84-87 (June 1983), and U.S. Pat. Nos. 4,686,631 and 5,148,458 issued to Ruud (the above-referenced documents are herein incorporated by reference). The PSSD has been successfully employed for residual stress measurement of materials ranging from thick plate weldments to ceramic coatings and to copper alloy strip moving at near 400 fpm. In addition, the PSSD has been used for texture studies in metals, simultaneous stress, and phase analysis of bearing steels (see U.S. Pat. No. 5,148,458 issued to Ruud), and for alloy composition measurements in platings. This invention differs from that in U.S. Pat. No. 5,148,458 in that the radial position of the detectors is varied depending upon the position of the x-ray source focus, the angle of the incident x-ray beam to the plating and substrate surface, the diffraction angles and the orientations of the diffracting crystallographic planes to the specimen surfaces. Due to the fiber optics, the PSSD is very versatile and can be made compact and portable. This coupled with the real-time analysis capability of the PSSD renders it well-suited for on-line commercial applications.
Taken together, the PSSD and other prior art devices have been used to analyze compositions based on the peak positions and intensities appearing in the diffraction patterns for given samples. They have been used for residual stress measurement, but not all are well suited for this specialized purpose. Also, they have been used for basic single phase analysis of the composition of a stoichiometric compound as described previously.
However, platings and coatings often comprise multiple layers and, therefore, multiple phases. Preferred crystallite orientation of the multiple phases complicate the process of analysis based only on the peak positions and intensities as from a conventional diffraction pattern. Moreover, the substrate often diffuses into the coating, and this further complicates any attempted XRD analysis based solely on peak intensities from planes of a single orientation to the sample surface. For example, with galvanneal plated cold rolled steel sheet, the substrate diffuses into the zinc during the plating process and forms intermetallic and solid solution phases which may adversely affect subsequent processing. These phases can result in inconsistent and/or adverse behavior (i.e., cracking, flaking, and powdering) during the forming operation. Unfortunately, the use of XRD with conventional scanning and/or PSDs is usually too slow and the instrumentation too delicate to be used during plating processes. Hence, there is currently no reliable way to assure the quality of galvanized steel and other platings, and quality control of such platings is therefore hampered.
There would be great commercial advantage if the PSSD could be adapted to yield a real-time detailed analysis of samples having polycrystalline platings and, specifically, for real-time analysis of the following characteristics:
phase composition (including degree of alloying); PA1 plating thickness; PA1 phase composition of platings having elements in common with adjacent platings and/or the substrate; PA1 crystalline phase depth; PA1 crystallinity and grain size; PA1 crystalline strain; PA1 preferred crystalline orientation; and PA1 simultaneous measurement of two or more of the above characteristics.